Bus support construction



p 1947- R. R. PITTMAN BUS SUPPORT CONSTRUCTION Filed ,May 5, 1946 Fig.3

INVENTOR Patented Sept. 23, 1947 UNITED STATES PATENT OFFICE BUS SUPPORT CONSTRUCTION Ralph R. Pittman, Park Hill, Ark.

Application May 3, 1946, Serial No.-666,902

10 Claims. 1

This invention relates to excess voltage protective structures for use in limiting the voltage which may be superimposed on an insulating bus support and subsequently extinguishing a power are which may follow an impulse discharge caused by the superimposed voltage.

Among the objects of the invention may be noted (a) the provision of multiplegaps in which the division of voltage between a plurality of serially arranged gaps remains substantially constant regardless of rate of change of applied volt- 'age or of weather conditions; (1)) the provision of a sparkover path including a plurality of gaps so combined with an insulating bus support that the sum of the voltages appearing across the individual gaps exceeds the voltage across the bus support; the provision of arc extinguishing means so combined with the plurality of gaps that the total length of the arc path within the i dividual arc interrupting 'devices may be much longer than that within a single arc interrupter bridging the support, thereby facilitating are extinguishment without sacrifice of protective ability and (d) the provision of a construction which may be simply and economically combined with any bus support which may be divided into three or more serially related insulating sections. Other objects and advantages will appear from the following description.

The invention is exemplified by the combination and relation of parts shown in the accompanying drawing and described in the specification, in which is illustrated and described a simple embodiment of the invention, and the scope of the invention is particularly pointed out in the appended claims.

Fig. l of the drawing is an'elevation of an embodiment of the invention; Fig. 2 shows one type of arc interrupting device suitable for use in the invention; and Fig. 3 is a circuit diagram showing advantageous relationship of certain electrical quantities inherent in the invention.

The protection of bus supports from excessive voltage, such as may be caused by lightning, has

been long recognized as important because failure of a bus support usually renders an entire generating or substation useless until replaced. Heretofore, the degree of protection has been determined by the sparkover voltage of a series of arc gaps connected to directly bridge the support from line to ground, certain of the arc gaps being utilized to prevent the passage of leakage current from line to ground through the remainder of the gaps, the latter constituting the arc extinguishing gaps. The total length of the individual gaps is fundamentally a criterion of the protective ability of the arrangement, and consequently this length is made as short as possible without destroying the arc extinguishing ability of the structure. However, arc extinguishing ability basically require a definite length of arc path, usually too long for the degree of protective ability required, since the latter basically requires a short arc path. Because of the considerations, devices heretofore have been a .compromise, there having been available no constructionwhich inherently provides the long .arcpath necessary for a-rc extinguishment without some sacrifice of protective ability. Existing constructions, employing simple series gaps from line to ground have bee-nmodified in various ways by changes in capacity relationships and other means, but the series circuit itself, simply bridging the equipment to be protected, without more,is inherently subjectto the above-mentioned limitations.

If, however, the voltage across apiece of insulating material is subdivided, and the gaps subjected to certain combinations of the subdivided voltages, it is possible to provide a construction having *high degree of protective ability along with a relatively long are path. The manner in'which this is accomplished by use of the present invention will "be clear from the following description, from which it will appear that the length of are desirable for successful are extinguishment maybe-easily attained along with surprisingly low sparkover voltage, thereby affording a high degreeof protection not only to the bus support itself, but to contiguous connected apparatus.

In the embodiment of the invention illustrated in Fig. 1,'-a' commonly-.usedbus support, comprising a stack of three insulator units, respectively numbered HL H, and 12, is supported on a lally column 21 of conducting material, the latter being connected in accordance with-common practice, to the ground-zflthrough the conductor 28.

A bus 15, of conducting material, is fastened to the top of the upper insulator unit 10 by means support between the :cap'of the'insulator ll and the pin of'the insulator 1-0., and held in position by means of the bolts '36,=extends' outwardly from the bus'support'below the bus -15, and the insulator unit I3 bridges the space between the -meniber 2| and the bus :l-5, thereby being connected electrically in :parallel "with the upper end or top insulator unit 10. Similarly, the insulator unit M is electrically connected in parallel with the lower endor bottom insulator unit 1'2, beingsupported between th'e outwardly extending conducting members "22 and 23, which are mounted on ,the .bus supports, respectively, be-

tween the pin of the insulator unit I I and the cap of the insulator unit I2, and between the pin of the insulator l2 and the top of the grounded lally column 21.

The conducting member 22 extends outwardly beyond the insulator unit 14, and mounted thereon is the first arc interrupter 24, the latter extending upwardly toward and in spaced relationship with the adjustable electrode ll of the bus to provide the leakage gap 18 therebetween. The conducting member 22 extends also on the other side of the bus support to provide a support for the second arc interrupter 25, the latter extending upwardly toward and in spaced relationship with the adjustable electrode ll of the conducting member 2| to provide the leakage gap l9 therebetween. In similar manner, the third arc interrupter 28 is mounted on the conducting member 23 to provide the gap between the adjustable electrode l1 and the end of the outwardly extending member 22 and the top of the arc interrupter 26.

In Fig. 2 is shown a well known and commonly used type of arc interrupter which is suitable for use in the structure, but it will be understood that there are other types of arc interrupters in which are extinguishing ability is a function of the length of the internal discharge column which may also be used. For illustrative purposes, the arc interrupter 26 is shown mounted on the conducting member 23 by means of the nut 33, the latter being threaded on the vent nipple 32. The tube 3|, of some gas-evolving insulating material such as hard fiber, is threadedly fastened to the vent nipple 32 at the lower end of the tube, and the cap threadedly engages the upper end of the tube and provides a closure therefor. The cap 30 is provided with a portion of conducting material which extends downwardly into the bore of the tube 3|, thereby establishing that the weakest electrical path from the cap 30 to the vent nipple 3| is internally through the bore of the tube. The cap 30 and the nipple 32 thus provide respectively upper and lower spaced insulated conducting electrodes of the arc interrupter between which a discharge may occur when a predetermined voltage is impressed across the electrodes. Following sparkover, the arc interrupter interrupts the follow current are by cooling as a result ofgas evolution from the inner wall of the tube.

The circuit diagram of Fig. 3 illustrates the particular set of voltage conditions which result from the application of a voltage E to the embodiment of the invention shown in Fig. 1. The capacity relationships of the similar insulators of Fig. 1 are, on inspection, such that capacitance of the parallel arrangement of the upper insulator units l0 and I3 is equal to the parallel arrangement of the lower insulator units [2 and I4, and twice the capacitance of single middle insulator unit H, and these capacitances are designated by the letters A, B, and C. Since the voltage distribution along a serially connected group of condensers varies inversely with the capacitance of the individual condensers, it is apparent that the voltage across the middle unit B is twice that of either A or C. Since the gap I8 and are interrupter 24 bridge the series arrangement of condensers A and B, there is normally impressed thereacross a voltage of E. Similarly E is impressed across the series arrangement of the gap 20 and the arc interrupter 26, and /2 E across the gap l9 and arc interrupter 25. The sum of the voltages applied to the sparkover paths is E plus E plus /g E or 2 E. These relation- 4 ships hold very closely for any rate of applicatiom of voltage between the bus l5 and the ground 29.

For best operation, the dielectric strength of the series arrangement of the gap I8 and the arc' interrupter 24 should be equal to the dielectric: strength of the gap 20 and the arc interrupter 26, and 150% of the dielectric strength of the series arrangement of the gap l9 and the arc interrupter 25. Under this condition, when a. predetermined sparkover voltage appears betweenthe bus I5 and ground 2!] simultaneous sparkover of all of the gaps will occur, and the sparkoverpath traversed by power current following spark-- over is from the bus l5 downwardly across the leakage gap l8 and through the arc interrupter 24 to the conducting member 22, thence upwardly through the arc interrupter 25 and the leakage gap H! to the conducting member 2|, thencedownwardly across the leakage gap 20 and the arc interrupter 26 to the conducting member 23, and thence through the lally column 21 and grounding conductor 28 to the ground 29. It isv apparent that the sparkover voltage of each portion of the fiashover path must be less than the withstand voltage of the associated portion of the bus support, to assure discharge through the arc interrupters instead of over the surface of insulators.

A stack of three insulator units, as here illustrated, is common construction for 115 kv. bus systems, and in the construction described above, an additional insulator unit has been added in parallel with the top unit and the bottom units of the stack. The capacitance of a commonly used insulator unit is about 40 mmf., so that the capacitance relationship from top to bottom is mmf., 40 mmf. and 80 mmf. Under normal kv. conditions, the voltage across the gap l8 and the associated arc interrupter 24 is of the normal line to ground voltage of 64 kv., or 48 kv. Similarly, the normal voltage across the gap 20 and the associated arc interrupter 26 is also 48 kv., and the normal voltage across the gap l9 and the associated arc interrupter 25 is of 64 kv., or 32 kv. The leakage gaps are therefore set for this particular application to withstand the normal voltage conditions. The voltage relationships hold for all practical purposes for any rate of voltage application, so that it is only necessary to determine the voltage which the bus support or some piece of connected apparatus will withstand without failure, and coordinate the components of the sparkover path at A, and of this value. Assuming that this value is 500 kv. at a 1% x 40 wave (an impulse reaching crest voltage in 1% microseconds and decaying to crest voltage in 40 microseconds), 3'75 kv. will be applied to the first and third sparkover paths, and 250 kv. to the intermediate sparkover paths. The total available voltage to sparkover the various gaps involved is 1000 kv., or twice the amount available if a simple sparkover path direct from bus to ground were used, as is now common practice. The construction here shown may therefore employ a total length of arc path twice that of the conventional construction, thereby greatly facilitating arc extinguishment, because after sparkover only a power are due to 64 kv. exists, and the extinguishment of this are is a matter of a more rapid increase in dielectric strength of arc path at current zero than in system recovery voltage.

If only three units are used in the construction, the voltage division is obviously and /3 of the applied voltage, so that the length of arc path may be increased 66 /3% over that of a azdirect path frombus to ground. Various other arrangements are .immediately apparent, .e. -.g.,

a-stack of four insulatorunits, dividing -the-line to ground voltagein four equal parts, andbridging the top three units, the bottom three units, :1 and the two intermediate units will also-yield. :a

conducting members. .This arrangement ispreferable to minimize corona effects, which may be deleterious to the arc interrupters, since-it results in maximum voltage stress being expressed across the leakage gaps, thereby holding all parts of the arc interrupters as near to ground potential as possible under normal voltage conditions The above described voltage conditions -may of course be obtained with simple open gaps, thereby enabling the use of a-longer total sparkover path from line to ground than is possible with a simple bus-to-ground arrangement, but under usual practical conditions, it is not ,possible to establish a sufficientlylong arc path to be self-extinguishing in open air. The construction therefore is most valuable when used to provide a long are path through suitable varc extinguishing devices without sacrifice of the low initial sparkover value necessary to assure excess-voltage protection to bus support and associated electrical apparatus.

In the above description, only capacitancerelationships have been mentioned. Strictly speaking, the voltage divides across the units from bus to ground in accordancewith the-impedance of the respective portions of the stack. For all practical purposes, however, the capacitance is the controlling component of impedance under lightning conditions, since with rapid application of voltage the current through aresistance is a direct function of the voltage, while the average current-through a capacitance is a function of rate of change of voltage and expressed by the relationship Where I equals average current in amperes. C equals capacitance in mlcromlcrofarads. dc

dt equals rate of change in kilovolts 'per microsecond.

For a 20 mmf. bus support, as here illustrated, at a rate of voltage application of 1000 kv./ms.,

From the above considerations, it will be seen that the voltage distribution at the rapid, or most I equals equals 20 amperes "destructive voltageapplications, is predominately controlled by the capacitance, and littleaffected 'by contamination by dirt and moisture deposits on the surface of the insulators; also that the more rapid the voltage rise, the less is the effect of the resistance component of impedance.

The foregoing description of the embodiment of the invention illustratedis intendedas-illusinterposition therebetween of a stack of at least three' mechanicallyjoined and serially connected -insulator units,- of a first are interrupter interposed between said bus and the junction of the lower and middle insulator units, a second arc interrupter interposed between'said junction and the junction-of the upper and middle insulator units, andathird arc interrupter interposed betweensaid last-named junction and said grounded member, said are interrupters embodying spaced conducting electrodes arranged to'p'rovide-a-preferential sparkover path therethrough .from saidbus to said grounded member.

-'2. The combination witha bus supported on a grounded member and insulated therefrom by interposition therebetween of a stack of at least three mechanically joined and serially connected insulator units, of a first arc interrupter interposed between said bus and the junction of the 'loWer and middle insulator units, a second are interrupterlinterpose'd between said junction and 'the junction of the upper and middle insulator units, and a third arc interrupter interposed between said last-named junction and said grounded member, said are interrupters embodying'nor- -mally insulated conducting electrodes arranged to provide a sparkover path from said bus to said grounded member, said sparkover path constituting a series dielectric circuit forming the weakest electrical path from said bus to said grounded member.

3. The combination with a bus supported on "a'grounded member and insulated therefrom to "a predetermined withstand voltage by interposition therebetween'of a stack of at least three mechanically joined and serially connected insulator units, of a first arc interrupter interposed between-said bus and the junction ofthe lower and middle insulator unit, a second arc interrupter interposed between said junction and "the junction of the upper and middle insulator unit; and a third are interrupter interposed between said last-named junction and said grounded member, said are interrupters embodying nor- *mally insulated-conductin electrodes arranged to provide a spar-kover path from said bus to said-grounded member when the voltage therebetween attains a predetermined sparkover voltage'less-than said predetermined withstand volt- "age.

4. In combination with a bus support including *a's't'ackof at leastthree-mechanically joined and seriallyconnected insulator units, spaced con- "ducting elements mounted on said insulator units, "saidelements constituting in series arrangement a-sparkover path in shunt with said bus support, said sparkover path including at least one spark gap in shunt with all insulator units abovethe bottom unit, atleast one spark gap inshunt with an insulator-units between the top and bottom unitaan'd at leastone spark gap in shunt with all insulator-units below the top unit, said con- 'ducting elements being so arranged that the di- -electric strength of said sparkover path is -less than the dielectric strength of said bus support.

'5. Incombinationwith a bus support including -'a*stack of at least three mechanically joined and serially connected insulator units, spaced conducting elements mounted on said insulator units, said elements constituting a, dielectric circuit in parallel with said bus support and adapted to sparkover and carry current when a predeter- -mined voltage'is applied across said bus support, said circuit including at least one spark gap bridging all insulator unit above the bottom unit, at least one spark gap bridging all units positioned between the top and bottom units, and at least one spark gap bridging all units below the top unit.

6. The combination with a bus support which is formed by mechanicall joining in series circuit relationship upper, intermediate and lower capand-pin type insulator units, of outwardly extending conducting members mounted on said support in spaced relationship to one another and to said bus, said members including a first conducting member mounted on said bus support at the juncture of the pin of the upper unit with the cap of the intermediate unit, a second conducting member mounted on said bus support at the junc ture of the pin of the intermediate unit with the cap of the lower unit, a third conducting member mounted on said bus support at the pin of the lower insulator unit, an upper arc interrupter mounted on said second member and extending upwardly therefrom in spaced relation with said bus, an intermediate arc interrupter mounted on said second member and eXtending upwardly therefrom in spaced relation with said first conducting member, and a lower arc interrupter mounted on saidthird conducting member and extending upwardly therefrom in spaced relation with said first conducting member, each of said arc interrupters constituting normally open circuit means in series with its associated gap between the upper end of the interrupter and the respective conducting member, the construction and arrangement of said are interrupters being such that the weakest electrical path across the insulator stack is serially through said are interrupters.

7. In combination with a bus support of insulating material having a predetermined withstand voltage, conducting means mounted thereon for dividing said support into at least three serially arranged sections, said conducting means constituting a dielectric circuit in parallel with said bus support and adapted to sparkover and carry current when a predetermined sparkover voltage less than said withstand voltage is applied across said bus support, said circuit including a spark gap between one end of said support and the adjacent end of the section at the other end of said support, a spark gap between the other end of said support and the adjacent end of the section at said one end of said support, a spark gap between the respective adjacent ends of the end sections of said support, and normally open circuit means in series with said gaps for interrupting a power arc following sparkover of said gaps.

8. In combination with a bus support of insulating material having a predetermined withstand voltage, conducting means mounted thereon for dividing said support into at least three serially arranged sections such that the capacitances of the respective end sections are equal and each individually at least as great as the capacitance of the intermediate section, said conducting means constituting a dielectric circuit in parallel with said bus support and adapted to sparkover and carry current when a predetermined sparkover voltage less than said withstand voltage is impressed across said bus support, said circuit including a spark gap between one end of said support and the adjacent end of the section at the other end of said support, a spark gap between the other end of said support and the adjacent end of the section at said one end of said support, a spark gap between the respective adjacent ends of the end sections of said support, and normally open circuit means in series with said gaps for interrupting a power are following sparkover of said gaps.

9. In combination with a bus support of insulating material having a predetermined withstand voltage, conducting means mounted thereon for dividing said support into at least three serially arranged sections such that the capacitances of the respective end sections are equal and the capacitance of each end section is greater than the capacitance of the intermediate section, said conducting means constituting a dielectric circuit in parallel with said bus support and adapted to sparkover and carry current when predetermined sparkover voltage less than said withstand voltage is impressed across said bus support, said circuit including a spark gap between one end of said support and the adjacent end of the section at the other end of said support, a spark gap between the other end of said support and the adjacent end of the section at said one end of said support, a spark gap between the respective adjacent ends of the end sections of said support, and normally open circuit means in series with said gaps for interrupting a power are following sparkover of said gaps.

10. The combination with a bus supported on a grounded member and insulated therefrom by interposition therebetween of a stack of at least three mechanically joined and serially connected insulator units, of a first are interrupter interposed between said bus and the upper end of the lower insulator of said stack, a second arc interrupter interposed between the upper end of the lower insulator of the stack and the lower end of the upper insulator of said stack, and a third are interrupter interposed between the lower end of the upper insulator of said stack and said grounded member, said are interrupters having ability to interrupt arcs therein when the total length of all of the arcs within the respective arc interrupters exceeds a predetermined magnitude and embodying normally insulated conductin electrodes between which are may occur to provide a sparkover path from said bus to said grounded member, said sparkover pathconstituting a series dielectric circuit which is the weakest electrical path from said bus to said grounded member.

RALPH R. PI'ITMAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,531,971 Peek Mar. 31, 1925 1,783,052 Peek Nov. 25, 1930 2,135,352 Rorden Nov. 1, 1938 1,849,862 Bailey Mar. 15, 1932 2,282,905 Towne May 12, 1942 FOREIGN PATENTS Number Country Date 40,108 Switzerland Apr. 22, 1908 36,708 Austria Mar. 26, 1909 

